In a few cases, the kernel will use pprint to put a diagnostic on the
standard error (file descriptor 2).
One of those is a warning that the process has used more than 100 file
descriptors.
That message is possibly obsolete and could be removed, but there are
others, such as
notifying an uncaught trap that are probably helpful to make visible.
In any case, as things stand, a busy exportfs might have many file
descriptors open,
provoking the diagnostic. Unfortunately, aux/listen and aux/listen1 connect
file
descriptor 2 to the incoming network connection. If the connection's
protocol is
not a simple, unstructured, textual one, diagnostics on the standard error
will cause confusion, in particular to devmnt.c if 9p is used.

/rc/bin/service files that start applications that run special protocols
might want to
redirect file descriptor 2; alternatively, perhaps aux/listen shouldn't
redirect fd 2 by
default: the few commands that do connect the remote user to shells, or
equivalent, including
telnetd and sshd could dup 1 to 2 when that was sensible.